plan: export the plan with data in bytes (not string) (python3)

For Python3, we need to open the plan data from Swift as bytes so it can
be written.

Also see Ia2108f3f64e3ae043cd050b5c3748682d0a816de

Change-Id: I380a7c86133d6ea4ff23bccc9acf79f01f07fada

tripleoclient/v1/overcloud_plan.py

@@ -203,5 +203,5 @@ class ExportPlan(command.Command):
         f = request.urlopen(tempurl)
         tarball_contents = f.read()
 
-        with open(outfile, 'w') as f:
+        with open(outfile, 'wb') as f:
             f.write(tarball_contents)